1. Field of the Invention
This invention relates to blood gas calibration and control fluids and, more particularly, fluids containing hemoglobin.
2. Description of the Prior Art
Blood gas tests are frequently used in health care facilities to determine abnormalities in pulmonary function. These parameters commonly used in determining pulmonary abnormalities are blood pH, P.sub.CO.sbsb.2 and P.sub.O.sbsb.2. The tests are performed by drawing blood from the patient and introducing this blood into specialized equipment which determines the various parameters to be measured. The specialized equipment must be calibrated frequently to determine if the readouts on values for patient blood are accurate. This calibration involves introducing various solutions or gases having predetermined amounts of constituents which are present in in vivo blood.
After the testing equipment has been calibrated it is necessary to assure maintenance of the calibration routine. For this reason the instrument is frequently tested with blood gas controls which quickly and readily determine any unexpected analytical deviations. The blood gas controls do not have absolute values for pH, P.sub.CO.sbsb.2 and P.sub.O.sbsb.2. Instead ranges are provided; if the instrument responds within the specified range, the accuracy of the calibration is assured.
Typical blood gas control techniques and solutions are taught in U.S. Pat. No. 3,859,049; Clin. Chem. 24, p. 793-795, 1978 by Steiner et al.; U.S. Pat. No. 3,973,913; and U.S. Pat. No. 4,001,142.
In calibrating or controlling of the instrumentation for blood gas analysis, it is desirable to use a material containing the predetermined amount of blood constituents which most approximates the function of blood to obtain accurate readings. Thus, the ideal solution would be blood having predetermined amounts of constituents to calibrate the instrumentation. However, use of blood as a calibration or control fluid is impractical because of instability and degradation problems upon aging, and the generation of methemoglobin on aging which destroys the oxygen carrying function of the hemoglobin constituents. Furthermore, with whole blood it is difficult if not impossible to adjust the pH accurately and reproducibly.
Thus, several other types of solutions have been proposed and used as blood gas calibration standards or controls. The most widely used are aqueous solutions such as taught in U.S. Pat. No. 4,001,142. These solutions have several problems associated with their use. Aqueous controls have excellent shelf life. However, once the seal is broken the aqueous control must be used immediately since when the aqueous control solution is open to the environment, there is rapid exchange of the dissolved gases and the control fluid with the external environment thereby changing the concentration of the assayed constituents. Such exchanges result in inaccurate recovery of the assayed parameters.
In addition, a small amount of liquid which has been exposed to, and therefore, equilibrated with the atmosphere, or other gas is present in the instrument sample chamber. The gas permeable membranes separating the electrode from the chamber also may contain trapped gas. This residue can mix with the sample and cause changes in the gaseous constituents, especially oxygen. With whole blood these changes are effectively negated, due to the reversible interaction of hemoglobin and oxygen which provides a high "oxygen buffer capacity". Typical aqueous blood gas controls contain no such "oxygen buffer capacity" and are, therefore, easily contaminated during sample handling and within the sample chamber.
The values obtained for the assayed constituents in whole blood (pH, P.sub.CO.sbsb.2 and P.sub.O.sbsb.2) are dramatically affected by the temperature of the sample chamber in which the sample is being analyzed. The control material must, therefore, respond to temperature changes in the instrument similar to whole blood. No typical aqueous blood gas control responds appropriately to temperature induced value changes in all measured parameters. The control material of the present invention containing stroma-free hemoglobin closely simulates the response of whole blood with respect to all measured parameters.
To avoid some of these problems treated red cells in a buffer have been used as a blood gas control. Louderback U.S. Pat. No. 3,973,913 teaches such a blood control standard in which the cells are treated with aldehyde to render the cell membrane less sensitive to lysing and inhibit the metabolism of the cell. The stability of the control when opened is improved but the shelf life is significantly decreased. However, the treatment effects the hemoglobin so that it is apparently no longer physiological. In order to provide an effective oxygen buffer system, the hemoglobin contained in the control must be in such a form that its oxygen affinity is similar to that of whole blood. The controls containing treated erythrocytes contain hemoglobin that has an enhanced oxygen affinity and, therefore, suffer from a reduced oxygen buffer capacity.
A quality control material for blood gas analysis must also be convenient to use and thus minimize pre-analytical variables. If it is extremely difficult to prepare the ampulized sample for analysis, then the potential for error increases. Controls containing treated red cells must be carefully re-equilibrated at 37.degree. C. In addition, the treated red cell controls may only be kept re-equilibrated for 30 minutes before discarding.
Thus, it is desirable to use a calibration solution which functions as nearly as possible to blood while eliminating the problems associated with the use of blood as a blood gas control or calibration fluid. In accordance with the present invention, a blood gas control fluid is provided which approximates the function and the gas carrying ability of human blood without the stability problems associated with blood. Like whole blood, the blood gas reference fluid of the present invention incorporates hemoglobin as a reversible oxygen buffer system thus minimizing any errors due to contamination during handling and analysis. The stroma-free hemoglobin has near normal oxygen affinity. The control fluid re-equilibrates at normal room temperature and may be re-equilibrated for up to 48 hours before assay without deleterious effects.
A further advantage of the present invention is the deep red color of the hemoglobin which allows easy visualization within the sample chamber of the analyzer. Like whole blood, this property readily shows the presence of any trapped air bubbles accidently introduced into the measuring chamber.